1. Field of the Invention
This invention relates to the completion of wells in subterranean formations, and more particularly to an apparatus for gravel-packing a void space exterior to a perforate liner installed in a well.
2. Description of the Prior Art
Recovering fluids, such as oil, gas and water, through a well from an unconsolidated or loosely consolidated subterranean formation often results in the undesirable flow of sand and other earth particles into the well. The particles can plug the flow channels of the formation and/or the well, and when entrained in the recovered fluid the particles can cause severe erosion of the metal surfaces of equipment, such as the production string, valves, pumps and flow lines, with which it comes in contact. Moreover, treatment of the produced fluid is generally required to remove the sand. Similar problems can occur during the injection of various fluids through a well into unconsolidated or loosely consolidated formations.
One conventional technique for completing a well in an incompetent formation so as to substantially prevent entrainment of earth particles into the well involves placing a perforate liner in the well at the lower end of a cemented casing string or well liner, and to thereafter pack gravel of selected size around the exterior of the perforate liner in the annular void space between the perforate liner and the walls of the borehole. The gravel can by hydraulically placed in the void space by circulating a suspension of the gravel in water or other liquid through the void space so that the gravel is deposited therein. Conventionally, the perforate liner is run into the well on a gravel-packing tool suspended from a running-in- string. The gravel-packing tool includes a crossover tool, a port collar, a check valve, and a stringer pipe. The crossover tool provides a first flow passage from the interior of the running-in string via the port collar to the void space surrounding the perforate liner, and a second flow passage from the interior of the stringer pipe through the check valve to the annulus above the tool between the running-in string and the well casing or well liner. The crossover tool is suitably packed off above and below the port collar. The stringer pipe is suspended below the crossover tool and check valve and extends to a point just above the lower end of the perforate liner. In operation, the gravel suspension is pumped down the running-in string and through the crossover tool and then outwardly through the gravel ports in the port collar into the void space between the walls of the borehole and the upper end of the perforate liner, whereupon the suspension flows downwardly through the annulus surrounding the perforate liner. The inflowing gravel suspension displaces the carrier liquid of the preceding suspension through the perforations in the perforate liner and into the lower end of the stringer pipe. The carrier fluid passes upwardly through the stringer pipe and check valve, outwardly through the crossover tool and then upwardly through the well annulus surrounding the running-in string above the gravel-packing tool for return to the earth surface.
Ideally, the suspension thus pumped into the upper end of the void space between the perforate liner and the formation wall will gradually move downwardly displacing the liquid already in this annulus inwardly through the perforations in the perforate liner. As the suspension reaches the bottom of the void space, the carrier liquid also passes through the perforations in the perforate liner, progressively leaving the gravel compactly packed in the void space surrounding the perforate liner, from the bottom of the well upward, until no more gravel suspension can be pumped into the upper end of the void space. Unfortunately, this process often does not operate as desired, even in substantially vertical wells, resulting in the void space surrounding the perforate liner being only partially filled with gravel. One of the theories for such failure is that bridging of the gravel between the perforate liner and the formation wall occurs at a point located a substantial distance above the top of the previously placed gravel bed, thus blocking further downward flow of the gravel suspension. After such a bridge occurs, the liquid in the suspension delivered into the void space above the bridge escapes inwardly through the perforate liner. The void space is progressively packed with gravel from the bridge upward leaving a gap in the gravel pack below the bridge. In practice, a number of such bridges may occur in long perforate liners, leaving a like number of gaps in the gravel pack.
Another problem encountered in the gravel-packing of wells in size segregation of gravel within the annulus. Since gravity is a major factor in the formation of a uniform gravel pack, the larger, more dense particles tend to settle faster, which can result in a particle size segregation in the gravel pack.
These problems have been largely overcome by the use of the gravel-packing tools disclosed in U.S. Pat. Nos. 3,637,010 and 3,741,301 to Maly and Robinson and in U.S. Pat. No. 3,802,500 to Schmidt. These tools include a plurality of flexible, radial baffles slidably mounted at spaced positions along the stringer pipe such that the gravel-packing tool is rotatable and axially movable within the perforate liner, within a limited distance, independent of the baffles. The baffles are slightly larger in diameter than the inside diameter of the perforate liner and therefore they offer substantial residence to the flow of fluid past the baffles in the perforate liner. These gravel-packing tools have been used successfully in the gravel-packing of perforate liners having internal diameters which are substantially smaller than the internal diameter of the casing string and/or well liner in the upper portion of the well.
When the perforate liner to be gravel-packed has substantially the same internal diameter as the casing string or well liner extending from the perforate liner to the earth surface, such as a perforate liner attached to a production casing, the gravel-packing tool is normally also provided with a bypass tool, such as a sliding valve, for bypassing the check valve when the gravel-packed tool is retrieved from the well. During retrieval of the gravel-packing tool the bypass tool would normally allow the fluid in the well annulus above the gravel-packing tool to bypass the check valve and drain through the annulus between the stringer pipe and the casing string. However, the presence of one or more baffles on the stringer pipe serves to severely restrict flow through this annulus and as a result the baffles would effectively swab the well fluids from the casing string as the baffled stringer pipe is retrieved. As is known, the swabbing of a well can result in a blowout or other dangerous well conditions. Since the use of a baffled stringer pipe is highly preferred for the gravel-packing of all wells, a need exists for a gravel-packing tool having a baffled stringer pipe which can be safely retrieved without swabbing the casing string through which it must pass.
Accordingly, a primary object of the present invention is to provide a gravel-packing tool with which a compact uniform bed of gravel can be hydraulically placed in a void space exterior to a perforate liner disposed in a well adjacent an incompetent formation.
Another object of this invention is to provide a gravel-packing tool having a baffled stringer pipe which can be safely withdrawn from the well after completion of the gravel-packing operation.
Yet another object of this invention is to provide a gravel-packing tool having a baffled stringer pipe which is suitable for forming a gravel pack around a perforate liner having substantially the same internal diameter as the casing string or well liner which extends from the perforate liner to a point near or at the earth surface.
Further objects, advantages and features of the invention will become apparent to those skilled in the art from the following description taken in conjunction with the accompanying drawings.